The present invention relates to cellular radio telecommunications systems, and more particularly to cellular radio telecommunications systems that employ a code division multiple access (CDMA) scheme.
Cellular radio telecommunications systems employ one or more channel access schemes. One well-known channel access scheme is the code division multiple access (CDMA) scheme. CDMA is well-known in the art. Unlike other channel access schemes (e.g., time division or frequency division multiple access), a number of different traffic channel signals are simultaneously transmitted in such a way that they overlap in both the time domain and the frequency domain.
In order to distinguish each traffic channel signal from the other traffic channel signals, each traffic channel signal is encoded with one or more unique spreading codes, as is well-known in the art. By modulating each of the traffic channel signals with a spreading code, the sampling rate (i.e., the "chip rate") may be substantially increased in accordance with a spreading factor. For example, each traffic channel signal is modulated in accordance with a digital modulation scheme, e.g., a quadrature amplitude modulation (QAM) or a phase shift keying (PSK) technique. Consequently, an in-phase and quadrature component signal is produced for each traffic channel signal. QAM and PSK are well known in the art. The in-phase and quadrature component signals associated with each of the traffic channels are then encoded using a unique spreading code sequence. The resulting in-phase and quadrature component signal pairs are sampled (i.e., at the chip rate) and individually weighted. The in-phase and quadrature component signals are eventually combined to form a composite in-phase signal and a composite quadrature signal. The composite in-phase signal and the composite quadrature signal are then separately filtered by a low-pass, pulse shaping filter. Subsequent to filtering, the composite in-phase signal and the composite quadrature signal are modulated by a cosine-carrier and a sine-carrier respectively and combined into a single, multicode CDMA signal. The single, multicode CDMA signal is then upconverted by a carrier frequency and the signal power associated with the CDMA signal is boosted by a high power amplifier prior to transmission. At a receiving unit, the baseband signal associated with each of the traffic channel signals is extracted from the CDMA signal by demodulating and decoding the CDMA signal using the carrier frequency and the various spreading codes. Furthermore, it will be understood that in a typical cellular telecommunications system, the transmission source may, for example, be a high power base station, and the receiving entity may, for example, be a mobile station (i.e., a mobile telephone).
When there is an especially large number of traffic channel signals, it is sometimes preferable to generate two or more CDMA signals, wherein each of the two or more CDMA signals is upconverted by its own unique CDMA carrier frequency. The two or more upconverted CDMA signals are then independently amplified by a corresponding high power amplifier prior to transmission, or alternatively, the two or more upconverted CDMA signals are combined into a single, CDMA signal, which is then amplified by a single, high power amplifier prior to transmission.
As one skilled in the art will readily appreciate, CDMA substantially increases system bandwidth, which in turn, increases the network's traffic handling capacity as a whole. In addition, combining independent CDMA signals into a single CDMA signal, as described above, is advantageous in that a single high power amplifier is required rather than a separate high power amplifier for each independent CDMA signal. This is advantageous because high power amplifiers are expensive, and employing one high power amplifier in place of many will result in a substantial cost savings.
Despite the advantages associated with CDMA, combining multiple traffic channel signals and/or independent CDMA signals, in general, significantly increases the peak-to-average power ratio associated with the resulting CDMA signal. More specifically, the peak-to-average power ratio for a CDMA signal can be determined in accordance with the following relationship: EQU PR.sub.PTA =PR.sub.F +10*log(N)
wherein PR.sub.PTA represents the peak-to-average power ratio of the corresponding composite signal, PR.sub.F represents the power ratio of the low-pass, pulse shaping filter and N represents the number of traffic channels which make up the CDMA signal.
The problem associated with large peak-to-average power ratios is that it diminishes the efficiency of the high power amplifier in the transmitter. Efficiency, as one skilled in the art will readily understand, is measured in terms of the amount of output power (i.e., Pmean) divided by the amount of input power (i.e., Pdc+Ppeak). As Ppeak (i.e., peak power) increases relative to Pmean, the efficiency of the high power amplifier decreases.
One possible solution is to simply limit or clip the amplitude (i.e., Ppeak) of the CDMA signal. Unfortunately, this is likely to result in the generation of intermodulation products and/or spectral distortions. Intermodulation products and/or spectral distortions are, in turn, likely to cause interference between the various traffic channel signals. Accordingly, this is not a preferred solution.
Another possible solution is to design a more complex high power amplifier, one that can tolerate and more efficiently amplify CDMA signals that exhibit large peak-to-average ratios. However, this too is not a preferred solution as the cost of high power amplifiers are generally proportional to complexity. Accordingly, this solution would result in driving up the cost of the telecommunications device that houses the high power amplifier.
U.S. Pat. No. 5,621,762 ("Miller et al.") offers yet another possible solution for the peak-to-average power ratio problem, that is to limit the peak-to-average power ratio before the soon-to-be transmitted telecommunications signal is filtered and subsequently amplified. More specifically, Miller describes a peak power suppression device for reducing the peak-to-average power ratio of a single code sequence at the input of the high power amplifier. The peak power suppression device employs a digital signal processor (DSP) which receives the single code sequence, maps the code sequence onto a symbol constellation diagram, predicts an expected response from the pulse shaping filter and limits the amplitudes appearing on the symbol constellation diagram in accordance with the expected response of the pulse shaping filter.
The primary problem with the solution offered in Miller is that peak power suppression device is designed for a non-CDMA application. Therefore, the peak power suppression device described therein is incapable of coping with the specific characteristics associated with CDMA, such as, high data bit rates, multiple traffic channel signals and/or multi-code sequences, and multiple CDMA carrier signals. For example, the peak power suppression device described in Miller is inherently slow, as evidenced by the fact that it employs a DSP, and by the fact that the DSP has the time necessary to execute a pulse shaping filter prediction algorithm. Therefore, a need exists for a telecommunications signal amplitude limitation device that is capable of limiting the peak-to-average power ratio of a telecommunications signal before it is filtered and subsequently amplified, and additionally, is capable of handling significantly higher data bit rates, multiple code sequences, and multiple CDMA carrier signals.